1. Field of the Invention
The invention relates to methods of, and apparatus for, stabilizing the output of a mode-locked laser which is used for optical communication, optical measurements, etc.
2. Description of the Related Art
Mode-locked lasers have many attractive features such as generation of optical pulses with high repetition frequency and short pulse duration. Therefore, these lasers have been actively investigated in order to apply them to the fields such as ultra-high speed and long distance optical communication, optical measurements, etc.
FIG. 17 A shows a conventional ring cavity-type mode-locked laser. The conventional ring cavity-type mode-locked laser which is shown in this figure consists of an optical modulator 102 to modulate the optical loss or phase with the specified frequency, a power supply 101 of the optical modulator 102, an optical amplifier 103 that amplifies the modulated optical signal, an optical isolator 104 that prescribes a direction of the propagation of optical signal and blocks off reflected light from optical components, an optical coupler 105 that outputs the amplified optical signal from the laser cavity, a wavelength tunable filter 106, and optical waveguides 107 that couple each of the above components optically (reference: H. Takara et al., "20 GHz transform-limited optical pulse generation and bit-error-rate operation using a tunable, actively mode-locked Er-doped fiber ring laser", Electron. Lett., vol. 29, No. 13, pp. 1149-1150, 1993).
As the optical modulator 102, a modulator which utilizes electro-optical effect of LiNbO.sub.3, etc., is mainly used. As the wavelength tunable filter 106, a dielectric multiple-film filter is mainly used. As the optical waveguide 107, an optical fiber is mainly used. As the optical amplifier 103, a rare earth-doped fiber amplifier which is doped with rare earths such as Er and Nd, semiconductor laser amplifier, etc., are mainly used.
Referring to FIGS. 17B and 17C, operation principles of the conventional mode-locked laser will be explained. FIG. 17B shows a typical spectral characteristic by the mode-locking, and FIG. 17C shows its time characteristics.
As shown in FIG. 17A, the optical modulator 102, the optical amplifier 103, the optical isolator 104, and the optical coupler 105 are coupled together into a ring shape with the optical waveguides 107 to form a ring cavity. When the physical length and the refractive index of components of the ring cavity are "h" and "n", the optical path length L of the ring cavity is defined as the summation of products of each refractive index n.sub.i and each physical length h.sub.i as in the following formula (where "i" is a natural number). EQU L=.SIGMA.h.sub.i n.sub.i ( 1)
For a ring cavity, multiple longitudinal modes exist with frequency intervals that are given by the fundamental cavity frequency f.sub.c =c/L (c is the velocity of light). When applying optical modulation with the following repetition frequency f.sub.m with the optical modulator 102 in the ring cavity, EQU f.sub.m =N.multidot.f.sub.c (N is an integer equal to or greater than one)(2)
a mode-locked oscillation is established, in that all longitudinal modes at frequency intervals of N.multidot.f.sub.c) have their phases aligned as shown in FIG. 17B, and an optical pulse train with the repetition period of 1/(N.multidot.f.sub.c) is obtained as shown in FIG. 17C. This formula (2) expresses a mode-locking condition.
The pulse width corresponds to the reciprocal of the oscillation spectral width d.nu. which is determined with the envelope of multiple longitudinal mode spectra, and the center of this spectral envelope is a central wavelength (optical frequency .nu..sub.0).
In addition, when the optical path length of the cavity L changes due to a temperature change, etc. the above-described longitudinal mode frequency interval f.sub.c also changes. Therefore, to fulfill the mode-locking condition according to the above formula (2), the modulation frequency f.sub.m must be changed in response to the change of f.sub.c. That is, to achieve a mode-locking condition without being influenced by the change of the cavity length, the modulation frequency f.sub.m must be changed in response to the change of the cavity optical path length. However, generally, because the modulation frequency f.sub.m was fixed, it was difficult to respond to a change of the cavity optical path length due to the temperature change, etc., and previous mode-locked lasers, such as those described above, were not practical.
To solve this problem, some methods for stabilizing the output of a mode-locked laser have been proposed by, for example, X. Shan (reference: X. Shan, et al., "Stabilising Er fiber soliton laser with pulse phase locking", Electron. Lett., vol. 28, No. 2, pp. 182-184, 1992).
FIG. 18 is a schematic diagram of this conventional method. In this conventional method, the phase difference between a mode locked frequency component S.sub.m1 from power supply 201 and a mode locked frequency component S.sub.m2 of the electrical signal that was obtained from a laser output by photoelectric conversion, and the cavity optical path length is controlled by an optical delay line 208 in the cavity to suppress the change of the phase difference. However, in this conventional method, because the stability of the laser output mainly depended on the initialization of the cavity optical path length, considerably precise adjustments of the cavity optical path length were required at the beginning of laser operation. Also, because it was necessary to change the cavity length when changing a mode locked frequency, the mode locked frequency could not be easily changed. Moreover, this method has a problem of low reliability because it does not monitor the stability of the laser output directly.
For example, when, the temperature, etc., changes the propagation length of the mode locking frequency component S.sub.m1, from power supply 201 to phase comparator 241, the cavity optical path length is also changed to maintain the phase difference of the mode locking frequency component to be constant, and the laser operation shifts out of the mode-locking condition and the laser output becomes unstable. Also, because the output frequency of the power supply is generally more than 2 GHz, high-speed operation is required for photoelectric converter 210, phase comparison receptacle 241, and peripheral circuits thereof.